Field of the Invention
The present invention relates to improvements in processes and apparatus for producing synthesis gas, or syngas, from light hydrocarbon gas such as methane or natural gas by the oxidation thereof. Such syngas, comprising a mixture of carbon monoxide and hydrogen, is useful for the preparation of a variety of other valuable chemical compounds, such as by application of the Fischer-Tropsch process.
The combustion stoichiometry of methane gas at 1000.degree. F. is highly exothermic and produces CO.sub.2 and H.sub.2 O according to the following reaction: EQU CH.sub.4 +20.sub.2 .fwdarw.CO.sub.2 +2H2O (-190.3 kcal/g mol CH.sub.4)
The formed gases are not useful for the production of valuable chemical compounds, and the high temperatures generated present problems with respect to reactors and catalysts which would be required to produce valuable products from the formed gases. It is known to produce useful gases, known as synthesis gases or syngases, by partial oxidation of methane and other light hydrocarbon gases, by steam or CO.sub.2 -reforming of methane and other light hydrocarbon gases, or by some combination of these two chemistries. The partial oxidation reaction of methane is a less highly exothermic reaction which, depending upon the relative proportions of the methane and oxygen and the reaction conditions, can proceed according to the following stoichiometry: EQU 2CH.sub.4 +20.sub.2 =2CO+2H.sub.2 +2H.sub.2 O (-64 kcal/g mol CH.sub.4) EQU 2CH.sub.4 +1.50.sub.2 =2CO+3H.sub.2 +1H.sub.2 O (-34.9 kcal/g mol CH.sub.4) EQU 2CH.sub.4 +O.sub.2 =2CO+4H.sub.2 +0H.sub.2 O (-5.7 kcal/g mol CH.sub.4).
It is most desirable to enable the partial oxidation reaction to proceed according to the latter reaction in order to produce the most valuable syngas and minimize the amount of heat produced, thereby protecting the apparatus and the catalyst bed, and to reduce the formation of steam, thereby increasing the yield of hydrogen and carbon monoxide and enabling the steam-reforming reaction to convert any steam and hydrogen into useful syngas components.
Conventional syngas-generating processes include the gas phase partial oxidation process (GPOX), the autothermal reforming process (ATR), the fluid bed syngas generation process (FBSG), the catalytic partial oxidation process (CPO) and various processes for steam reforming. Each of these processes has advantages and disadvantages when compared to each other.
The GPOX process, illustrated for example by U.S. Pat. No. 5,292,246; UK Application GB 2,202,321A and EPO Application 0 312,133, involves the oxidation of the feed hydrocarbon gaseous, liquid or solid form, in the gas phase rather than on a catalyst surface. The individual components are introduced at a burner where they meet in a diffusion flame, which produces over-oxidation and excessive heat generation. The gas may be preheated and pressurized, to reduce the reaction time.
The ATR process and the FBSG process involve a combination of gas phase partial oxidation and steam reforming chemistry.
In the ATR process, illustrated for example by U.S. Pat. No. 5,492,649 and Canadian Application 2,153,304, the hydrocarbon feed and the oxygen feed, and optionally steam, are heated, and mixed at the outlet of a single steam, are heated, and mixed at the outlet of a single large coaxial burner or injector which discharges into a gas phase oxidation zone. The gases are reacted in the gas phase in the partial oxidation combustion zone, and then flow into a large bed of steam reforming catalyst, such as large catalyst pellets, or a monolithic body, to complete steam reforming. The entire hydrocarbon conversion is completed by a single reactor aided by internal combustion. The burner is the key element because it mixes the feedstreams in a turbulent diffusion flame. The reaction products are introduced to the fixed bed catalyst zone, preferably of large catalyst pellets, at high temperatures from the combustion zone, due to the over-oxidation which occurs in the diffusion flame of the burner, where the oxygen and hydrocarbon gas meet. The diffusion flame includes oxygen-rich and hydrocarbon-rich zones. These result in both complete combustion and substantially higher temperatures, in the oxygen-rich zones, and hydrocarbon cracking and soot-formation, in the hydrocarbon-rich zones.
In the ATR process, the gases are intended to react before they reach the catalyst, i.e., the oxidation chemistry occurs in the gas phase, and only the steam reforming chemistry occurs in the catalytic bed. In fact, long residence times are required because diffusion flames are initiated with a large amount of over-oxidation, accompanied by a large amount of heat. Thus, time is required for the relatively slow, endothermic gas phase steam reforming reactions to cool the gas enough for introduction into the catalyst bed to prevent thermal damage to the catalyst.
In the FBSG process illustrated for example by U.S. Pat. Nos. 4,877,550; 5,143,647 and 5,160,456, the hydrocarbon gas, such as methane, and oxygen or an oxygen-supplying gas are introduced separately into a catalyst fluid bed for mixing therewithin. While the gases may be introduced at a plurality of sites, to more evenly distribute the gases over the inlet of the fluid bed of the reactor, the fact that the gases mix within the fluid bed results in over-oxidation hot spots and catalyst sintering or agglomeration due to the oxygen concentration being higher and closer to full-combustion stoichiometry in areas closest to the oxygen injection sites. The gas phase partial oxidation and steam reforming chemistry employed in the FBSG and the Autothermal Reforming (ATR) process have very similar material balance when using similar feed. However, ATR is limited in size by the scaleability of its injector design, and the more-scaleable FBSG is economically debited by the cost of fluid solids and dust cleanup and by the expense of replacing agglomerated and/or eroded catalyst. The dust comprises catalyst fines due to catalyst attrition in the bed, and these fines are expensive to clean out of the syngas. While the chemistry is correct, these two processes have significant drawbacks. Both require very large reactors. For FBSG there is a significant expense in fluid solids management. For Autothermal Reforming there is a large and problematic methane/oxygen feed nozzle.
CPO (catalytic partial oxidation) attempts to eliminate the gas phase partial oxidation reactions entirely, and instead perform all of the partial oxidation reactions on a highly active catalyst (usually Rh) to convert the hydrocarbon catalytically at such a high rate or low dwell time that the gas phase reactions, or combustion stoichiometry, never have the opportunity to occur. It is crucial that the gases fed to a CPO catalyst be thoroughly premixed in order to avoid gas phase reactions which damage the catalyst, reduce its activity and promote non-complete combustion reactions. Also, while more selective than gas phase POX, CPO catalysts currently known have not exhibited such high levels of steam reforming activity that would permit them to reform over-oxidized feeds at the high space velocities employed in CPO. Thus, it is especially critical in CPO to avoid non-selective gas-phase oxidation, and therefore it is especially important to provide premixed feed, which is slower to begin gas phase chemistry. Also it is especially important to provide the premixed feed at high temperature and velocity to enable the catalytic reaction of the premixed gases at short contact times. However, it is dangerous to premix heated methane and oxygen and it is difficult to avoid gas phase reactions between these gases, which proceed at undesirable combustion stoichiometry to produce steam and carbon dioxide.
For catalytic partial oxidation (CPO), while certain metals can catalyze the desired oxidation chemistry at very short contact times, it is necessary to premix the methane and oxygen gases at high temperature, pressure and velocity to enable the catalytic reaction to proceed at short contact times in reduced scale reactors, and so that the chemistry occurs at the correct stoichiometry throughout the catalytic zone. The use of catalyst-impregnated monoliths can catalyze the desired chemistry with residence times below about 0.05 sec. When compared to conventional ATR reactors, FBSG reactors and GPOX reactors, this represents more than a one hundred fold decrease in residence time and, therefore, in residence volume. However, such a reactor is unworkable without a means to premix CH.sub.4 or other hydrocarbon and O.sub.2 at high temperature, pressure, and velocity, safely and while avoiding gas phase reactions that are not within the desired partial oxidation zone and/or in contact with the catalyst. In other words, the catalytic partial oxidation process has the potential to provide extraordinary reactor productivity in view of the extremely high space velocities of the throughput if the aforementioned problems are avoided.
It is known that successful operation of the catalytic partial oxidation (CPO) process on a commercial scale requires high conversion of the hydrocarbon feedstock at high hourly space velocities, using preheated mixtures of oxygen gas and methane in a preferred ratio of about 1:2, or 0.5, and under elevated pressures. Reference is made to Jacobs et al. U.S. Pat. No. 5,510,056 (Shell) for its disclosure of such a process.
The problems with such known processes are that they are dangerous, since pre-formed preheated mixtures of oxygen and methane, at pressures of interest for syngas production, e.g., 10 atmospheres or more, are co-reactive and explosive, and any gas stage reaction or autoignition prior to introduction to the reaction zone, results in combustion stoichiometry which is highly exothermic and produces catalyst sintering.
It has been proposed to conduct a high efficiency catalytic partial oxidation (CPO) process using pre-formed mixtures of high temperature, high pressure methane and oxygen gases and steam at space velocities up to 500,000 hr.sup.-1, using a mixing and distributing means having a plurality of mixing tubes within which the gases are mixed prior to discharge through a multi-disc catalyst stack. Reference is made to EPO 303,438, assigned to Davy McKee Corp., which discloses a high temperature, high pressure partial oxidation process, and a mixing and distribution catalyst bed apparatus for producing a gaseous reaction product comprising methane, carbon oxides, hydrogen and steam in the absence of a reforming reaction. The preheated methane and oxygen gases are combined in the mixing tubes, through small orifices, and are discharged from the tubes at a distance downstream of the orifices sufficient to produce mixtures of the gases prior to discharge from diverging nozzles which reduce the velocity of the gas mixture at the inlet to the partial oxidation catalyst zone.
The mixing and distribution means of EPO 303,438 is ineffective in enabling the desired stoichiometry, i.e., 2CH.sub.4 +O.sub.2 .fwdarw.2CO+4H.sub.2 +OH.sub.2 O, to produce the most useful syngas to the exclusion of other than very small amounts of CO.sub.2, H.sub.2 O and CH.sub.4. This appears to be due to the fact that such mixing and distributing means is inadequate and allows the heated methane and oxygen to co-exist in the gaseous state, upstream of the partial oxidation catalyst zone, for too long a residence time, such as more than about 9 milliseconds, so that the methane and oxygen initiate non-catalytic reaction in the gaseous state to produce the wrong or undesirable stoichiometry, resulting in the production of steam and CO.sub.2, reduced amounts of H.sub.2 and CO and high heat generation which can result in catalyst sintering or agglomeration and waste, and damage to the apparatus.
Furthermore, the control of pressure drop through the mixing and distributing means appears to be inadequate. Specifically, EPO 303,438 and the related WO 90-06282 disclose a fuel stream pressure drop of 0.0% of downstream pressure, while related WO 90-06281 discloses a fuel stream pressure drop of 2.2% of downstream pressure and related WO 90-06297 is silent regarding fuel pressures. Inadequate control of pressure drop through the mixing and distributing means results in reactor instability and in nozzle-to-nozzle variations in gas stoichiometry, which facilitates non-catalytic reaction in the gaseous state to produce the wrong or undesirable stoichiometry, facilitates hot spots and carbon deposition, and can result in catalyst agglomeration and waste, and damage to the apparatus.